The present invention relates to the manufacture of a material comprising a fiber reinforced thermoplastic resin composite, the material having a three dimensional non-planar form. The material is malleable, particularly by molding, stamping or heat shaping, into the desired final shape of an article of manufacture.
Fiber reinforced thermoplastic resin composites are a rapidly developing materials area. A basic advantage of thermoplastic composites over the thermosets is that they require no cure and can be shaped into articles by economic, high speed mass production methods.
One production method is to initially form, by the use of an aqueous slurry method well known to the art, a sheet having fibers and thermoset and/or thermoplastic powders interstitially mixed therein. The sheet is thereafter dewatered and dried, and thereafter formed into a three-dimensional article by stamping and/or heat forming operations.
For example, U.S. Pat. No. 4,645,565, assigned to Arjomarei-Prioux, teaches the manufacture of a material in sheet form wherein the sheet is prepared by a paper making technique. The sheet fabricated by this method comprises from 20 to 95% of reinforcing fibers and from 80 to 5% of thermoplastic resin in powder form forming the basic mixture and may furthermore include from 5 to 25% of a polyolefin pulp, from 5 to 30% of a binding agent, and from 0.2 to 10% of at least one flocculating agent.
U.S. Pat. No. 4,426,470, issued Jan. 17, 1984 to the Dow Chemical Company, also addresses the preparation of a reinforced polymer composite sheet having a uniform mix of fiber, polymer and binder wherein the sheet is prepared by forming dilute aqueous slurries of a solid heat fusible organic polymer, a reinforcing material and a bound charge latex binder.
U.S. Pat. No. 4,612,251, also assigned to Arjomarei-Prioux, issued Sept. 16, 1986, also teaches the fabrication of a sheet prepared according to a paper making process from a composition comprising about 45 to 140 parts by dry weight of a fibrous mixture, about 30 to 90 parts by dry weight of cellulosic fibers, and 15 to 50 parts by dry weight of mineral fibers per 100 parts by dry weight of a latex binder.
Great Britain Pat. No. 1,263,812, published Feb. 16, 1972, assigned to Wiggins-Teape Research and Development Ltd., teaches the fabrication of moldable sheet material by forming a paste by thoroughly mixing a natural or synthetic polymeric particulate solid with a binder, preparing a fibrous pulp, preparing a furnish by thoroughly mixing the paste with the fibrous pulp in a ratio by weight of solids of between 25% paste to 75% fiber and 75% paste to 25% fiber, draining the furnish on a foraminous screen, and therafter drying the furnish.
Unfortunately, the above patents merely teach the manufacture of a substantially planar two dimensional sheet which thereafter must be molded to form an article of the desired shape. The molding process usually involves the use of a stamping press, wherein the fibers located around the edges of the article or the sides thereof are damaged due to their abrupt displacement from an initially planar orientation. As a consequence, the fiber length in practical thermoplastic composite articles originally formed from sheets is usually less than 1 mm. Such short fibers understandably provide only for minimal reinforcement.
A manufacturing process that yields articles having improved reinforcement properties initially positions cellulosic fibers in their final "three dimensional" location prior to the stamping operation. The cellulosic fibers, (along with any thermoset and/or thermoplastic resin powders mixed therein), are carried by an aqueous slurry and deposited in their approximate final orientation on the surface of a three dimensional mold having a porous forming surface. The cellulosic fibers and powder particles are thereafter dewatered and dried prior to being stamped into their final orientation. Since the cellulosic fibers have already been approximately positioned in their final orientation, the stamping operation does not break a significant number of the cellulosic fibers.
Such a process may be studied in U.S. Pat. No. 3,157,566 or U.S. Pat. No. 3,271,239, wherein the cellulosic fibers and lesser amounts of certain thermoplastic resin particles are deposited by an aqueous medium on a three dimensional male and/or female mold.
Such a process would probably be inoperative if used to form a thermoplastic composite three dimensional article if the thermoplastic material, supplied to the tank in the form of a powder, were to have a specific gravity less than 1. The thermoplastic powder would tend to float upon the surface of the tank with subsequent powder rich areas being formed within the three dimensional composite structure. Unnecessarily complex binders and flocculating agents would have to be incorporated within the slurry to prevent the formation of articles having stratified reinforcing fiber locations.
A process need be developed wherein by the proper selection of thermoplastic materials a thermoplastic composite three dimensional article may be formed having homogeneous material properties throughout its entire cross section.